1. Field of the Invention
The present invention relates to a heterojunction bipolar transistor, and more particularly to a wide band-gap bipolar transistor having a base region of germanium.
2. Description of the Related Art Including Information Disclosed Under .sctn..sctn.1.97-1.99:
The so-called wide band-gap bipolar transistor has been reported in "Proceedings of the IEEE" Vol. 70, No. 1 (January 1982) pages 13 to 23, the article "Heterostructure Bipolar Transistors and Integrated Circuits" by Herbert Kroemer. It is a heterojunction transistor having an emitter made of a semiconductor having an energy band-gap wider than the base. The injection efficiency of minority carrier from emitter to base is improved to obtain an increased current gain.
An example of the wide band-gap bipolar transistor in the prior art has been proposed in "OYO BUTURI" published by the Japanese Society of Applied Physics, Vol. 54, No. 11 pages 1192 to 1197 and has a collector composed of an N.sup.+ -GaAs deposited on an insulating GaAs substrate and an N-GaAs formed on the N.sup.+ -GaAs, a base composed of P.sup.+ -GaAs and an emitter composed of an N-AlGaAs formed on the P-GaAs and an N.sup.+ -GaAs formed on the N-AlGaAs. AlGaAs has an energy band-gap of about 1.9 eV which is wider than GaAs of 1.42 eV. Thus, an improved electron injection efficiency and an increased current gain is obtained. Electron in GaAs has a mobility of about 4,600 cm.sup.2 /V.sec which is greater than Si, and has a high saturated electron drift velocity of about 1.8.times.10.sup.7 cm/sec. Thus, electrons run through the base and collector regions in a very short time, resulted in a high speed operation.
On the other hand, the wide band-gap bipolar transistor in the prior-art has many drawbacks. The energy band-gap of GaAs is wider than silicon of 1.12 eV. Due to this wide band-gap, the prior art transistor requires a high voltage signal for its electrical operation and consumes a large power, compared to a silicon transistor. Moreover, GaAlAs of emitter region and N-GaAs of collector region have a difficulty in forming a low-resistive ohmic contact with a metal. In the prior-art transistor, N.sup.+ -GaAs is interposed between the N-GaAlAs of emitter region and an emitter electrode of metal and between the N-GaAs of collector region and a collector electrode of metal for improving the ohmic contact property. Therefore, the device structure is complicated and causes a difficulty in manufacturing the same. Furthermore, GaAs is a semiconductor of direct transition type, while Si and Ge are an indirect transition type. Life time of minority carrier in the direct transition type semiconductor is shorter than in the indirect transition type semiconductor. Considerable part of electrons injected into the P.sup.+ -GaAs base region disappear by recombination. This disappearance lowers the injection efficiency of minority carriers injected into base region. From this point of view, it is preferable to use the indirect transition type semiconductor such as Si and Ce for the base region.
For improving some of the above-explained drawbacks, it has been proposed in "Proceedings of the IEEE" Vol. 70, No. 1, page 23 to use GaAs as a collector region, Ge as a base region and GaAs as an emitter region. Ge used as the base region has a low energy band-gap of 0.66 eV and a hole mobility of 4 or 5 times of GaAs or Si and is an indirect transition type semiconductor. Therefore, a low voltage drive, a low power consumption, a low base resistance and an improved injection efficiency of carrier are achieved.
However, as pointed out in "Proceedings of the IEEE" Vol. 70, No. 1, page 23, fifth paragraph of left side column, it is very hard to grow GaAs on Ge without interface defects. This GaAs-Ge-GaAs structure is not a practical resolution for the above-mentioned drawbacks. Moreover, the GaAs as the emitter region still remains the difficulty for obtaining a low-resistive ohmic contact with a metallic emitter electrode.